Conversion options for indexed-color images

When you convert an image to Indexed Color mode in Photoshop, you can specify options that determine how colors are reduced and displayed. These settings influence file size, image quality, and compatibility with formats like GIF and PNG-8.

Palette types

The palette defines which colors are available in the Indexed Color image. Different palette types are suited for different use cases:

• Exact: Uses the exact colors in the original image (only available if the image has 256 colors or fewer). Produces no dithering.
• System (Mac OS/Windows): This uses the system’s default 8-bit palette, which is based on uniform RGB sampling.
• Web: Uses the 216 web-safe colors to avoid browser dithering on older displays limited to 256 colors.
• Uniform: Samples colors evenly from the RGB cube (for example, six levels of red, green, and blue create 216 colors).
• Local (Perceptual, Selective, Adaptive): Creates a palette based on the current image’s colors.
• Perceptual prioritizes colors that the human eye is more sensitive to.
• Selective favors broad areas of color and preserves web colors.
• Adaptive samples the colors most common in the image.
• Master (Perceptual, Selective, Adaptive) – Similar to local palettes but considers all open documents instead of just one.
• Custom – Lets you edit or load a custom color table in the Color Table dialog box.
• Previous – Reuses the palette from the last indexed color conversion.

For Palette such as Uniform, Perceptual, Selective, or Adaptive, you can specify the exact number of colors (up to 256). Photoshop always treats the image as 8-bit, but this setting controls how the color table is built.

Color inclusion and transparency

You can control how specific colors and transparent areas are handled:

• Forced: This ensures that certain colors are always included in the palette. Options include Black and White, Primaries, Web, or a Custom selection.
• Transparency: Preserves transparent areas by adding a transparency entry to the palette. If deselected, transparent areas are filled with a matte color.
• Matte: Specifies the fill color for anti-aliased edges or transparent areas, helping them blend with a web background. Selecting None creates hard-edged transparency.

Dithering simulates colors not available in the palette by blending available pixels. While dithering can improve visual quality, it may increase file size. Options include:

• None: Uses the closest available color, producing sharp transitions and a posterized look.
• Diffusion: Uses error diffusion for smoother results. Preserve Exact Colors protects colors already in the palette.
• Pattern: Applies a halftone-like grid to simulate missing colors.
• Noise: Adds randomness to reduce seam patterns, useful when slicing images for web layouts.